source: draft-ietf-httpbis/latest/p4-conditional.xml @ 443

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[29]1<?xml version="1.0" encoding="utf-8"?>
[101]2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
[8]3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
[29]14  <!ENTITY ID-VERSION "latest">
[443]15  <!ENTITY ID-MONTH "February">
[439]16  <!ENTITY ID-YEAR "2009">
[424]17  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[205]18  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[306]20  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[31]21  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[163]22  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[31]23  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[45]26  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[205]27  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
[8]28]>
29<?rfc toc="yes" ?>
[29]30<?rfc symrefs="yes" ?>
31<?rfc sortrefs="yes" ?>
[8]32<?rfc compact="yes"?>
33<?rfc subcompact="no" ?>
34<?rfc linkmailto="no" ?>
35<?rfc editing="no" ?>
[203]36<?rfc comments="yes"?>
37<?rfc inline="yes"?>
[8]38<?rfc-ext allow-markup-in-artwork="yes" ?>
39<?rfc-ext include-references-in-index="yes" ?>
[308]40<rfc obsoletes="2616" category="std" x:maturity-level="draft"
[29]41     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
[153]42     xmlns:x='http://purl.org/net/xml2rfc/ext'>
[8]43<front>
44
[120]45  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
[8]46
[29]47  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
48    <organization abbrev="Day Software">Day Software</organization>
[8]49    <address>
50      <postal>
[29]51        <street>23 Corporate Plaza DR, Suite 280</street>
52        <city>Newport Beach</city>
[8]53        <region>CA</region>
[29]54        <code>92660</code>
55        <country>USA</country>
[8]56      </postal>
[29]57      <phone>+1-949-706-5300</phone>
58      <facsimile>+1-949-706-5305</facsimile>
59      <email>fielding@gbiv.com</email>
60      <uri>http://roy.gbiv.com/</uri>
[8]61    </address>
62  </author>
63
[29]64  <author initials="J." surname="Gettys" fullname="Jim Gettys">
65    <organization>One Laptop per Child</organization>
[8]66    <address>
67      <postal>
[29]68        <street>21 Oak Knoll Road</street>
69        <city>Carlisle</city>
[8]70        <region>MA</region>
[29]71        <code>01741</code>
72        <country>USA</country>
[8]73      </postal>
[29]74      <email>jg@laptop.org</email>
75      <uri>http://www.laptop.org/</uri>
[8]76    </address>
77  </author>
78 
79  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
[29]80    <organization abbrev="HP">Hewlett-Packard Company</organization>
[8]81    <address>
82      <postal>
[29]83        <street>HP Labs, Large Scale Systems Group</street>
84        <street>1501 Page Mill Road, MS 1177</street>
[8]85        <city>Palo Alto</city>
86        <region>CA</region>
[29]87        <code>94304</code>
88        <country>USA</country>
[8]89      </postal>
[29]90      <email>JeffMogul@acm.org</email>
[8]91    </address>
92  </author>
93
94  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
[29]95    <organization abbrev="Microsoft">Microsoft Corporation</organization>
[8]96    <address>
97      <postal>
[29]98        <street>1 Microsoft Way</street>
99        <city>Redmond</city>
100        <region>WA</region>
101        <code>98052</code>
102        <country>USA</country>
[8]103      </postal>
[29]104      <email>henrikn@microsoft.com</email>
[8]105    </address>
106  </author>
107
108  <author initials="L." surname="Masinter" fullname="Larry Masinter">
[29]109    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
[8]110    <address>
111      <postal>
[29]112        <street>345 Park Ave</street>
113        <city>San Jose</city>
[8]114        <region>CA</region>
[29]115        <code>95110</code>
116        <country>USA</country>
[8]117      </postal>
[29]118      <email>LMM@acm.org</email>
119      <uri>http://larry.masinter.net/</uri>
[8]120    </address>
121  </author>
122 
123  <author initials="P." surname="Leach" fullname="Paul J. Leach">
124    <organization abbrev="Microsoft">Microsoft Corporation</organization>
125    <address>
126      <postal>
127        <street>1 Microsoft Way</street>
128        <city>Redmond</city>
129        <region>WA</region>
130        <code>98052</code>
131      </postal>
132      <email>paulle@microsoft.com</email>
133    </address>
134  </author>
135   
136  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
137    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
138    <address>
139      <postal>
[34]140        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
141        <street>The Stata Center, Building 32</street>
142        <street>32 Vassar Street</street>
[8]143        <city>Cambridge</city>
144        <region>MA</region>
145        <code>02139</code>
[29]146        <country>USA</country>
[8]147      </postal>
148      <email>timbl@w3.org</email>
[34]149      <uri>http://www.w3.org/People/Berners-Lee/</uri>
[8]150    </address>
151  </author>
152
[95]153  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
[94]154    <organization abbrev="W3C">World Wide Web Consortium</organization>
155    <address>
156      <postal>
157        <street>W3C / ERCIM</street>
158        <street>2004, rte des Lucioles</street>
159        <city>Sophia-Antipolis</city>
160        <region>AM</region>
161        <code>06902</code>
162        <country>France</country>
163      </postal>
164      <email>ylafon@w3.org</email>
165      <uri>http://www.raubacapeu.net/people/yves/</uri>
166    </address>
167  </author>
[95]168
169  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
170    <organization abbrev="greenbytes">greenbytes GmbH</organization>
171    <address>
172      <postal>
173        <street>Hafenweg 16</street>
174        <city>Muenster</city><region>NW</region><code>48155</code>
175        <country>Germany</country>
176      </postal>
177      <phone>+49 251 2807760</phone>   
178      <facsimile>+49 251 2807761</facsimile>   
179      <email>julian.reschke@greenbytes.de</email>       
180      <uri>http://greenbytes.de/tech/webdav/</uri>     
181    </address>
182  </author>
183
[31]184  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
[440]185  <workgroup>HTTPbis Working Group</workgroup>
[8]186
187<abstract>
188<t>
189   The Hypertext Transfer Protocol (HTTP) is an application-level
190   protocol for distributed, collaborative, hypermedia information
[29]191   systems. HTTP has been in use by the World Wide Web global information
[35]192   initiative since 1990. This document is Part 4 of the seven-part specification
[29]193   that defines the protocol referred to as "HTTP/1.1" and, taken together,
[42]194   obsoletes RFC 2616.  Part 4 defines request header fields for
[29]195   indicating conditional requests and the rules for constructing responses
196   to those requests.
[8]197</t>
198</abstract>
[36]199
200<note title="Editorial Note (To be removed by RFC Editor)">
201  <t>
202    Discussion of this draft should take place on the HTTPBIS working group
203    mailing list (ietf-http-wg@w3.org). The current issues list is
[324]204    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
[36]205    and related documents (including fancy diffs) can be found at
[324]206    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
[36]207  </t>
[153]208  <t>
[382]209    The changes in this draft are summarized in <xref target="changes.since.05"/>.
[153]210  </t>
[36]211</note>
[8]212</front>
213<middle>
214<section title="Introduction" anchor="introduction">
215<t>
[163]216   This document defines HTTP/1.1 response metadata for indicating potential
217   changes to payload content, including modification time stamps and opaque
218   entity-tags, and the HTTP conditional request mechanisms that allow
219   preconditions to be placed on a request method.  Conditional GET requests
220   allow for efficient cache updates.  Other conditional request methods are
221   used to protect against overwriting or misunderstanding the state of a
222   resource that has been changed unbeknownst to the requesting client.
[8]223</t>
[163]224<t>
225   This document is currently disorganized in order to minimize the changes
226   between drafts and enable reviewers to see the smaller errata changes.
227   The next draft will reorganize the sections to better reflect the content.
228   In particular, the sections on resource metadata will be discussed first
229   and then followed by each conditional request-header, concluding with a
230   definition of precedence and the expectation of ordering strong validator
231   checks before weak validator checks.  It is likely that more content from
232   &caching; will migrate to this part, where appropriate.
233   The current mess reflects how widely dispersed these topics and associated
234   requirements had become in <xref target="RFC2616"/>.
235</t>
[96]236
237<section title="Requirements" anchor="intro.requirements">
238<t>
239   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
240   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
241   document are to be interpreted as described in <xref target="RFC2119"/>.
242</t>
243<t>
244   An implementation is not compliant if it fails to satisfy one or more
245   of the &MUST; or &REQUIRED; level requirements for the protocols it
246   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
247   level and all the &SHOULD; level requirements for its protocols is said
248   to be "unconditionally compliant"; one that satisfies all the &MUST;
249   level requirements but not all the &SHOULD; level requirements for its
250   protocols is said to be "conditionally compliant."
251</t>
[8]252</section>
253
[424]254<section title="Syntax Notation" anchor="notation">
[425]255  <x:anchor-alias value="ALPHA"/>
256  <x:anchor-alias value="CR"/>
257  <x:anchor-alias value="DIGIT"/>
258  <x:anchor-alias value="LF"/>
259  <x:anchor-alias value="OCTET"/>
260  <x:anchor-alias value="VCHAR"/>
261  <x:anchor-alias value="WSP"/>
[424]262<t>
263  This specification uses the ABNF syntax defined in &notation;.
[425]264  The following core rules are included by
265  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
266  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
267  DIGIT (decimal 0-9), DQUOTE (double quote),
268  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
269  OCTET (any 8-bit sequence of data), SP (space),
270  VCHAR (any visible USASCII character),
271  and WSP (whitespace).
[424]272</t>
273
274<section title="Core Rules" anchor="core.rules">
[229]275  <x:anchor-alias value="quoted-string"/>
[362]276  <x:anchor-alias value="OWS"/>
[205]277<t>
[424]278  The core rules below are defined in &basic-rules;:
[205]279</t>
280<figure><artwork type="abnf2616">
[229]281  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
[362]282  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
[205]283</artwork></figure>
[424]284</section>
285
286<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
[229]287  <x:anchor-alias value="HTTP-date"/>
[424]288<t>
[205]289  The ABNF rules below are defined in other parts:
290</t>
[207]291<figure><!--Part1--><artwork type="abnf2616">
[229]292  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
[205]293</artwork></figure>
294</section>
295
[424]296</section>
297
298</section>
299
[8]300<section title="Entity Tags" anchor="entity.tags">
[229]301  <x:anchor-alias value="entity-tag"/>
302  <x:anchor-alias value="opaque-tag"/>
303  <x:anchor-alias value="weak"/>
[8]304<t>
305   Entity tags are used for comparing two or more entities from the same
306   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
307   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
[29]308   If-Range (&header-if-range;) header fields. The definition of how they
[8]309   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
310   entity tag consists of an opaque quoted string, possibly prefixed by
311   a weakness indicator.
312</t>
313<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
[229]314  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
315  <x:ref>weak</x:ref>       = "W/"
316  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
[8]317</artwork></figure>
318<t>
319   A "strong entity tag" &MAY; be shared by two entities of a resource
320   only if they are equivalent by octet equality.
321</t>
322<t>
323   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
324   two entities of a resource only if the entities are equivalent and
325   could be substituted for each other with no significant change in
326   semantics. A weak entity tag can only be used for weak comparison.
327</t>
328<t>
329   An entity tag &MUST; be unique across all versions of all entities
330   associated with a particular resource. A given entity tag value &MAY;
331   be used for entities obtained by requests on different URIs. The use
332   of the same entity tag value in conjunction with entities obtained by
333   requests on different URIs does not imply the equivalence of those
334   entities.
335</t>
336</section>
337
[45]338<section title="Status Code Definitions">
339<section title="304 Not Modified" anchor="status.304">
340  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
341  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
342<t>
343   If the client has performed a conditional GET request and access is
344   allowed, but the document has not been modified, the server &SHOULD;
345   respond with this status code. The 304 response &MUST-NOT; contain a
346   message-body, and thus is always terminated by the first empty line
347   after the header fields.
348</t>
349<t>
350   The response &MUST; include the following header fields:
351  <list style="symbols">
[305]352    <x:lt>
[306]353      <t>Date, unless its omission is required by &clockless;.</t>
354      <t>
355         If a clockless origin server obeys these rules, and proxies and
[305]356         clients add their own Date to any response received without one (as
[306]357         already specified by &header-date;, caches will operate
[305]358         correctly.</t>
359    </x:lt>
360    <x:lt>
361      <t>ETag and/or Content-Location, if the header would have been sent
[306]362         in a 200 response to the same request.</t>
[305]363    </x:lt>
364    <x:lt>
365      <t>Expires, Cache-Control, and/or Vary, if the field-value might
[306]366         differ from that sent in any previous response for the same
367         variant.</t>
[305]368    </x:lt>
[45]369  </list>
370</t>
371<t>
[115]372   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
[45]373   the response &SHOULD-NOT;  include other entity-headers.
374   Otherwise (i.e., the conditional GET used a weak validator), the
375   response &MUST-NOT; include other entity-headers; this prevents
376   inconsistencies between cached entity-bodies and updated headers.
377</t>
378<t>
379   If a 304 response indicates an entity not currently cached, then the
380   cache &MUST; disregard the response and repeat the request without the
381   conditional.
382</t>
383<t>
384   If a cache uses a received 304 response to update a cache entry, the
385   cache &MUST; update the entry to reflect any new field values given in
386   the response.
387</t>
388</section>
389
390<section title="412 Precondition Failed" anchor="status.412">
391  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
392  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
393<t>
394   The precondition given in one or more of the request-header fields
395   evaluated to false when it was tested on the server. This response
396   code allows the client to place preconditions on the current resource
397   metainformation (header field data) and thus prevent the requested
398   method from being applied to a resource other than the one intended.
399</t>
400</section>
401</section>
402
[8]403<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
404<t>
405   Since both origin servers and caches will compare two validators to
406   decide if they represent the same or different entities, one normally
407   would expect that if the entity (the entity-body or any entity-headers)
408   changes in any way, then the associated validator would
409   change as well. If this is true, then we call this validator a
410   "strong validator."
411</t>
412<t>
413   However, there might be cases when a server prefers to change the
414   validator only on semantically significant changes, and not when
415   insignificant aspects of the entity change. A validator that does not
416   always change when the resource changes is a "weak validator."
417</t>
418<t>
419   Entity tags are normally "strong validators," but the protocol
420   provides a mechanism to tag an entity tag as "weak." One can think of
421   a strong validator as one that changes whenever the bits of an entity
422   changes, while a weak value changes whenever the meaning of an entity
423   changes. Alternatively, one can think of a strong validator as part
424   of an identifier for a specific entity, while a weak validator is
425   part of an identifier for a set of semantically equivalent entities.
426  <list><t>
427      <x:h>Note:</x:h> One example of a strong validator is an integer that is
428      incremented in stable storage every time an entity is changed.
429    </t><t>
430      An entity's modification time, if represented with one-second
431      resolution, could be a weak validator, since it is possible that
432      the resource might be modified twice during a single second.
433    </t><t>
434      Support for weak validators is optional. However, weak validators
435      allow for more efficient caching of equivalent objects; for
436      example, a hit counter on a site is probably good enough if it is
437      updated every few days or weeks, and any value during that period
438      is likely "good enough" to be equivalent.
439    </t></list>
440</t>
441<t>
442   A "use" of a validator is either when a client generates a request
443   and includes the validator in a validating header field, or when a
444   server compares two validators.
445</t>
446<t>
447   Strong validators are usable in any context. Weak validators are only
448   usable in contexts that do not depend on exact equality of an entity.
449   For example, either kind is usable for a conditional GET of a full
450   entity. However, only a strong validator is usable for a sub-range
451   retrieval, since otherwise the client might end up with an internally
452   inconsistent entity.
453</t>
454<t>
[245]455   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
[8]456</t>
457<t>
[172]458   The only function that HTTP/1.1 defines on validators is
[8]459   comparison. There are two validator comparison functions, depending
460   on whether the comparison context allows the use of weak validators
461   or not:
462  <list style="symbols">
463     <t>The strong comparison function: in order to be considered equal,
[298]464        both opaque-tags &MUST; be identical character-by-character, and both
465        &MUST-NOT; be weak.</t>
466     <t>The weak comparison function: in order to be considered equal, both
467        opaque-tags &MUST; be identical character-by-character.</t>
[8]468  </list>
469</t>
470<t>
[298]471   The example below shows the results for a set of entity tag pairs,
472   and both the weak and strong comparison function results:
473</t>
474<texttable align="left">
475  <ttcol>ETag 1</ttcol>
476  <ttcol>ETag 2</ttcol>
477  <ttcol>Strong Comparison</ttcol>
478  <ttcol>Weak Comparison</ttcol>
479
480  <c>W/"1"</c>
481  <c>W/"1"</c>
482  <c>no match</c>
483  <c>match</c>
484 
485  <c>W/"1"</c>
486  <c>W/"2"</c>
487  <c>no match</c>
488  <c>no match</c>
489
490  <c>W/"1"</c>
491  <c>"1"</c>
492  <c>no match</c>
493  <c>match</c>
494
495  <c>"1"</c>
496  <c>"1"</c>
497  <c>match</c>
498  <c>match</c>
499</texttable>
500<t>
[8]501   An entity tag is strong unless it is explicitly tagged as weak.
502   <xref target="entity.tags"/> gives the syntax for entity tags.
503</t>
504<t>
505   A Last-Modified time, when used as a validator in a request, is
506   implicitly weak unless it is possible to deduce that it is strong,
507   using the following rules:
508  <list style="symbols">
509     <t>The validator is being compared by an origin server to the
510        actual current validator for the entity and,</t>
511     <t>That origin server reliably knows that the associated entity did
512        not change twice during the second covered by the presented
513        validator.</t>
514  </list>
515</t>
516<t>
517   or
518  <list style="symbols">
519     <t>The validator is about to be used by a client in an If-Modified-Since
520        or If-Unmodified-Since header, because the client
521        has a cache entry for the associated entity, and</t>
522     <t>That cache entry includes a Date value, which gives the time
523        when the origin server sent the original response, and</t>
524     <t>The presented Last-Modified time is at least 60 seconds before
525        the Date value.</t>
526  </list>
527</t>
528<t>
529   or
530  <list style="symbols">
531     <t>The validator is being compared by an intermediate cache to the
532        validator stored in its cache entry for the entity, and</t>
533     <t>That cache entry includes a Date value, which gives the time
534        when the origin server sent the original response, and</t>
535     <t>The presented Last-Modified time is at least 60 seconds before
536        the Date value.</t>
537  </list>
538</t>
539<t>
540   This method relies on the fact that if two different responses were
541   sent by the origin server during the same second, but both had the
542   same Last-Modified time, then at least one of those responses would
543   have a Date value equal to its Last-Modified time. The arbitrary 60-second
544   limit guards against the possibility that the Date and Last-Modified
545   values are generated from different clocks, or at somewhat
546   different times during the preparation of the response. An
547   implementation &MAY; use a value larger than 60 seconds, if it is
548   believed that 60 seconds is too short.
549</t>
550<t>
551   If a client wishes to perform a sub-range retrieval on a value for
552   which it has only a Last-Modified time and no opaque validator, it
553   &MAY; do this only if the Last-Modified time is strong in the sense
554   described here.
555</t>
556<t>
[245]557   A cache or origin server receiving a conditional range request
558   (<xref target="Part5"/>) &MUST; use the strong comparison function to
[8]559   evaluate the condition.
560</t>
561<t>
562   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
563   retrievals on values that have been obtained from HTTP/1.0
564   servers.
565</t>
566</section>
567
568<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="rules.for.when.to.use.entity.tags.and.last-modified.dates">
569<t>
570   We adopt a set of rules and recommendations for origin servers,
571   clients, and caches regarding when various validator types ought to
572   be used, and for what purposes.
573</t>
574<t>
575   HTTP/1.1 origin servers:
576  <list style="symbols">
577     <t>&SHOULD; send an entity tag validator unless it is not feasible to
578        generate one.</t>
579
580     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
581        performance considerations support the use of weak entity tags,
582        or if it is unfeasible to send a strong entity tag.</t>
583
584     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
585        unless the risk of a breakdown in semantic transparency that
586        could result from using this date in an If-Modified-Since header
587        would lead to serious problems.</t>
588  </list>
589</t>
590<t>
591   In other words, the preferred behavior for an HTTP/1.1 origin server
592   is to send both a strong entity tag and a Last-Modified value.
593</t>
594<t>
595   In order to be legal, a strong entity tag &MUST; change whenever the
[302]596   associated entity changes in any way. A weak entity tag &SHOULD;
[8]597   change whenever the associated entity changes in a semantically
598   significant way.
599  <list><t>
600      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
601      origin server must avoid reusing a specific strong entity tag
602      value for two different entities, or reusing a specific weak
603      entity tag value for two semantically different entities. Cache
604      entries might persist for arbitrarily long periods, regardless of
605      expiration times, so it might be inappropriate to expect that a
606      cache will never again attempt to validate an entry using a
607      validator that it obtained at some point in the past.
608  </t></list>
609</t>
610<t>
611   HTTP/1.1 clients:
612  <list style="symbols">
613     <t>If an entity tag has been provided by the origin server, &MUST;
614        use that entity tag in any cache-conditional request (using If-Match
615        or If-None-Match).</t>
616
617     <t>If only a Last-Modified value has been provided by the origin
618        server, &SHOULD; use that value in non-subrange cache-conditional
619        requests (using If-Modified-Since).</t>
620
621     <t>If only a Last-Modified value has been provided by an HTTP/1.0
622        origin server, &MAY; use that value in subrange cache-conditional
623        requests (using If-Unmodified-Since:). The user agent &SHOULD;
624        provide a way to disable this, in case of difficulty.</t>
625
626     <t>If both an entity tag and a Last-Modified value have been
627        provided by the origin server, &SHOULD; use both validators in
628        cache-conditional requests. This allows both HTTP/1.0 and
629        HTTP/1.1 caches to respond appropriately.</t>
630  </list>
631</t>
632<t>
633   An HTTP/1.1 origin server, upon receiving a conditional request that
634   includes both a Last-Modified date (e.g., in an If-Modified-Since or
635   If-Unmodified-Since header field) and one or more entity tags (e.g.,
636   in an If-Match, If-None-Match, or If-Range header field) as cache
637   validators, &MUST-NOT; return a response status of 304 (Not Modified)
638   unless doing so is consistent with all of the conditional header
639   fields in the request.
640</t>
641<t>
642   An HTTP/1.1 caching proxy, upon receiving a conditional request that
643   includes both a Last-Modified date and one or more entity tags as
644   cache validators, &MUST-NOT; return a locally cached response to the
645   client unless that cached response is consistent with all of the
646   conditional header fields in the request.
647  <list><t>
648      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
649      servers and clients should transmit as much non-redundant
650      information as is available in their responses and requests.
651      HTTP/1.1 systems receiving this information will make the most
652      conservative assumptions about the validators they receive.
653  </t><t>
654      HTTP/1.0 clients and caches will ignore entity tags. Generally,
655      last-modified values received or used by these systems will
656      support transparent and efficient caching, and so HTTP/1.1 origin
657      servers should provide Last-Modified values. In those rare cases
658      where the use of a Last-Modified value as a validator by an
659      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
660      origin servers should not provide one.
661  </t></list>
662</t>
663</section>
664
665<section title="Header Field Definitions" anchor="header.fields">
666<t>
[117]667   This section defines the syntax and semantics of HTTP/1.1 header fields
668   related to conditional requests.
[8]669</t>
[117]670<t>
671   For entity-header fields, both sender and recipient refer to either the
672   client or the server, depending on who sends and who receives the entity.
673</t>
[8]674
675<section title="ETag" anchor="header.etag">
676  <iref primary="true" item="ETag header" x:for-anchor=""/>
677  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
[229]678  <x:anchor-alias value="ETag"/>
[362]679  <x:anchor-alias value="ETag-v"/>
[8]680<t>
[362]681   The response-header field "ETag" provides the current value of the
[321]682   entity tag (see <xref target="entity.tags"/>) for the requested variant.
683   The headers used with entity
[115]684   tags are described in Sections <xref target="header.if-match" format="counter"/>
685   and <xref target="header.if-none-match" format="counter"/> of this document,
686   and in &header-if-range;. The entity tag
[8]687   &MAY; be used for comparison with other entities from the same resource
688   (see <xref target="weak.and.strong.validators"/>).
689</t>
[362]690<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
[366]691  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
[362]692  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
[8]693</artwork></figure>
694<figure><preamble>
[362]695  Examples:
[8]696</preamble>
697<artwork type="example">
[362]698  ETag: "xyzzy"
699  ETag: W/"xyzzy"
700  ETag: ""
[8]701</artwork></figure>
[183]702<t>
703   The ETag response-header field value, an entity tag, provides for an
704   "opaque" cache validator. This might allow more reliable validation
705   in situations where it is inconvenient to store modification dates,
706   where the one-second resolution of HTTP date values is not
707   sufficient, or where the origin server wishes to avoid certain
708   paradoxes that might arise from the use of modification dates.
709</t>
710<t>
711   The principle behind entity tags is that only the service author
712   knows the semantics of a resource well enough to select an
713   appropriate cache validation mechanism, and the specification of any
714   validator comparison function more complex than byte-equality would
715   open up a can of worms. Thus, comparisons of any other headers
716   (except Last-Modified, for compatibility with HTTP/1.0) are never
717   used for purposes of validating a cache entry.
718</t>
[8]719</section>
720
721<section title="If-Match" anchor="header.if-match">
722  <iref primary="true" item="If-Match header" x:for-anchor=""/>
723  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
[229]724  <x:anchor-alias value="If-Match"/>
[362]725  <x:anchor-alias value="If-Match-v"/>
[8]726<t>
[362]727   The request-header field "If-Match" is used with a method to make it
[8]728   conditional. A client that has one or more entities previously
729   obtained from the resource can verify that one of those entities is
730   current by including a list of their associated entity tags in the
731   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
732   purpose of this feature is to allow efficient updates of cached
733   information with a minimum amount of transaction overhead. It is also
734   used, on updating requests, to prevent inadvertent modification of
735   the wrong version of a resource. As a special case, the value "*"
736   matches any current entity of the resource.
737</t>
[362]738<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
[366]739  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
[362]740  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
[8]741</artwork></figure>
742<t>
743   If any of the entity tags match the entity tag of the entity that
744   would have been returned in the response to a similar GET request
745   (without the If-Match header) on that resource, or if "*" is given
746   and any current entity exists for that resource, then the server &MAY;
747   perform the requested method as if the If-Match header field did not
748   exist.
749</t>
750<t>
751   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
752   to compare the entity tags in If-Match.
753</t>
754<t>
755   If none of the entity tags match, or if "*" is given and no current
756   entity exists, the server &MUST-NOT; perform the requested method, and
757   &MUST; return a 412 (Precondition Failed) response. This behavior is
758   most useful when the client wants to prevent an updating method, such
759   as PUT, from modifying a resource that has changed since the client
760   last retrieved it.
761</t>
762<t>
763   If the request would, without the If-Match header field, result in
764   anything other than a 2xx or 412 status, then the If-Match header
765   &MUST; be ignored.
766</t>
767<t>
768   The meaning of "If-Match: *" is that the method &SHOULD; be performed
769   if the representation selected by the origin server (or by a cache,
[29]770   possibly using the Vary mechanism, see &header-vary;) exists, and
[8]771   &MUST-NOT; be performed if the representation does not exist.
772</t>
773<t>
774   A request intended to update a resource (e.g., a PUT) &MAY; include an
775   If-Match header field to signal that the request method &MUST-NOT; be
776   applied if the entity corresponding to the If-Match value (a single
777   entity tag) is no longer a representation of that resource. This
778   allows the user to indicate that they do not wish the request to be
779   successful if the resource has been changed without their knowledge.
780   Examples:
781</t>
782<figure><artwork type="example">
[362]783  If-Match: "xyzzy"
784  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
785  If-Match: *
[8]786</artwork></figure>
787<t>
788   The result of a request having both an If-Match header field and
789   either an If-None-Match or an If-Modified-Since header fields is
790   undefined by this specification.
791</t>
792</section>
793
794<section title="If-Modified-Since" anchor="header.if-modified-since">
795  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
796  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
[229]797  <x:anchor-alias value="If-Modified-Since"/>
[362]798  <x:anchor-alias value="If-Modified-Since-v"/>
[8]799<t>
[362]800   The request-header field "If-Modified-Since" is used with a method to
[8]801   make it conditional: if the requested variant has not been modified
802   since the time specified in this field, an entity will not be
[137]803   returned from the server; instead, a 304 (Not Modified) response will
[8]804   be returned without any message-body.
805</t>
[362]806<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
[376]807  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
808                        <x:ref>If-Modified-Since-v</x:ref>
[362]809  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
[8]810</artwork></figure>
811<t>
812   An example of the field is:
813</t>
814<figure><artwork type="example">
[362]815  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
[8]816</artwork></figure>
817<t>
818   A GET method with an If-Modified-Since header and no Range header
819   requests that the identified entity be transferred only if it has
820   been modified since the date given by the If-Modified-Since header.
821   The algorithm for determining this includes the following cases:
822  <list style="numbers">
823      <t>If the request would normally result in anything other than a
824         200 (OK) status, or if the passed If-Modified-Since date is
825         invalid, the response is exactly the same as for a normal GET.
826         A date which is later than the server's current time is
827         invalid.</t>
828
829      <t>If the variant has been modified since the If-Modified-Since
830         date, the response is exactly the same as for a normal GET.</t>
831
832      <t>If the variant has not been modified since a valid If-Modified-Since
833         date, the server &SHOULD; return a 304 (Not
834         Modified) response.</t>
835  </list>
836</t>
837<t>
838   The purpose of this feature is to allow efficient updates of cached
839   information with a minimum amount of transaction overhead.
840  <list><t>
841      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
[29]842      see &header-range; for full details.
[8]843    </t><t>
844      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
845      clock might not be synchronized with the client.
846    </t><t>
847      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
848      servers will use an exact date comparison function, rather than a
849      less-than function, for deciding whether to send a 304 (Not
850      Modified) response. To get best results when sending an If-Modified-Since
851      header field for cache validation, clients are
852      advised to use the exact date string received in a previous Last-Modified
853      header field whenever possible.
854    </t><t>
855      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
856      header instead of a date taken from the Last-Modified header for
857      the same request, the client should be aware of the fact that this
858      date is interpreted in the server's understanding of time. The
859      client should consider unsynchronized clocks and rounding problems
860      due to the different encodings of time between the client and
861      server. This includes the possibility of race conditions if the
862      document has changed between the time it was first requested and
863      the If-Modified-Since date of a subsequent request, and the
864      possibility of clock-skew-related problems if the If-Modified-Since
865      date is derived from the client's clock without correction
866      to the server's clock. Corrections for different time bases
867      between client and server are at best approximate due to network
868      latency.
869    </t>
870  </list>
871</t>
872<t>
873   The result of a request having both an If-Modified-Since header field
874   and either an If-Match or an If-Unmodified-Since header fields is
875   undefined by this specification.
876</t>
877</section>
878
879<section title="If-None-Match" anchor="header.if-none-match">
880  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
881  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
[229]882  <x:anchor-alias value="If-None-Match"/>
[362]883  <x:anchor-alias value="If-None-Match-v"/>
[8]884<t>
[362]885   The request-header field "If-None-Match" is used with a method to make
[8]886   it conditional. A client that has one or more entities previously
887   obtained from the resource can verify that none of those entities is
888   current by including a list of their associated entity tags in the
889   If-None-Match header field. The purpose of this feature is to allow
890   efficient updates of cached information with a minimum amount of
891   transaction overhead. It is also used to prevent a method (e.g. PUT)
892   from inadvertently modifying an existing resource when the client
893   believes that the resource does not exist.
894</t>
895<t>
896   As a special case, the value "*" matches any current entity of the
897   resource.
898</t>
[362]899<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
[366]900  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
[362]901  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
[8]902</artwork></figure>
903<t>
904   If any of the entity tags match the entity tag of the entity that
905   would have been returned in the response to a similar GET request
906   (without the If-None-Match header) on that resource, or if "*" is
907   given and any current entity exists for that resource, then the
908   server &MUST-NOT; perform the requested method, unless required to do
909   so because the resource's modification date fails to match that
910   supplied in an If-Modified-Since header field in the request.
911   Instead, if the request method was GET or HEAD, the server &SHOULD;
912   respond with a 304 (Not Modified) response, including the cache-related
913   header fields (particularly ETag) of one of the entities that
914   matched. For all other request methods, the server &MUST; respond with
915   a status of 412 (Precondition Failed).
916</t>
917<t>
[245]918   See <xref target="weak.and.strong.validators"/> for rules on how to
919   determine if two entity tags match.
[8]920</t>
921<t>
922   If none of the entity tags match, then the server &MAY; perform the
923   requested method as if the If-None-Match header field did not exist,
924   but &MUST; also ignore any If-Modified-Since header field(s) in the
925   request. That is, if no entity tags match, then the server &MUST-NOT;
926   return a 304 (Not Modified) response.
927</t>
928<t>
929   If the request would, without the If-None-Match header field, result
930   in anything other than a 2xx or 304 status, then the If-None-Match
931   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
932   server behavior when both If-Modified-Since and If-None-Match appear
933   in the same request.)
934</t>
935<t>
936   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
937   performed if the representation selected by the origin server (or by
[29]938   a cache, possibly using the Vary mechanism, see &header-vary;)
[8]939   exists, and &SHOULD; be performed if the representation does not exist.
940   This feature is intended to be useful in preventing races between PUT
941   operations.
942</t>
943<t>
944   Examples:
945</t>
946<figure><artwork type="example">
[362]947  If-None-Match: "xyzzy"
948  If-None-Match: W/"xyzzy"
949  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
950  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
951  If-None-Match: *
[8]952</artwork></figure>
953<t>
954   The result of a request having both an If-None-Match header field and
955   either an If-Match or an If-Unmodified-Since header fields is
956   undefined by this specification.
957</t>
958</section>
959
960<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
961  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
962  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
[229]963  <x:anchor-alias value="If-Unmodified-Since"/>
[362]964  <x:anchor-alias value="If-Unmodified-Since-v"/>
[8]965<t>
[362]966   The request-header field "If-Unmodified-Since" is used with a method to
[8]967   make it conditional. If the requested resource has not been modified
968   since the time specified in this field, the server &SHOULD; perform the
969   requested operation as if the If-Unmodified-Since header were not
970   present.
971</t>
972<t>
973   If the requested variant has been modified since the specified time,
974   the server &MUST-NOT; perform the requested operation, and &MUST; return
975   a 412 (Precondition Failed).
976</t>
[362]977<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
[376]978  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
979                          <x:ref>If-Unmodified-Since-v</x:ref>
[362]980  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
[8]981</artwork></figure>
982<t>
983   An example of the field is:
984</t>
985<figure><artwork type="example">
[362]986  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
[8]987</artwork></figure>
988<t>
989   If the request normally (i.e., without the If-Unmodified-Since
990   header) would result in anything other than a 2xx or 412 status, the
991   If-Unmodified-Since header &SHOULD; be ignored.
992</t>
993<t>
994   If the specified date is invalid, the header is ignored.
995</t>
996<t>
997   The result of a request having both an If-Unmodified-Since header
998   field and either an If-None-Match or an If-Modified-Since header
999   fields is undefined by this specification.
1000</t>
1001</section>
1002
1003<section title="Last-Modified" anchor="header.last-modified">
1004  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1005  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
[229]1006  <x:anchor-alias value="Last-Modified"/>
[362]1007  <x:anchor-alias value="Last-Modified-v"/>
[8]1008<t>
[362]1009   The entity-header field "Last-Modified" indicates the date and time at
[8]1010   which the origin server believes the variant was last modified.
1011</t>
[362]1012<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
[366]1013  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
[362]1014  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
[8]1015</artwork></figure>
1016<t>
1017   An example of its use is
1018</t>
1019<figure><artwork type="example">
[362]1020  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
[8]1021</artwork></figure>
1022<t>
1023   The exact meaning of this header field depends on the implementation
1024   of the origin server and the nature of the original resource. For
1025   files, it may be just the file system last-modified time. For
1026   entities with dynamically included parts, it may be the most recent
1027   of the set of last-modify times for its component parts. For database
1028   gateways, it may be the last-update time stamp of the record. For
1029   virtual objects, it may be the last time the internal state changed.
1030</t>
1031<t>
1032   An origin server &MUST-NOT; send a Last-Modified date which is later
1033   than the server's time of message origination. In such cases, where
1034   the resource's last modification would indicate some time in the
1035   future, the server &MUST; replace that date with the message
1036   origination date.
1037</t>
1038<t>
1039   An origin server &SHOULD; obtain the Last-Modified value of the entity
1040   as close as possible to the time that it generates the Date value of
1041   its response. This allows a recipient to make an accurate assessment
1042   of the entity's modification time, especially if the entity changes
1043   near the time that the response is generated.
1044</t>
1045<t>
1046   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1047</t>
[183]1048<t>
1049   The Last-Modified entity-header field value is often used as a cache
1050   validator. In simple terms, a cache entry is considered to be valid
1051   if the entity has not been modified since the Last-Modified value.
1052</t>
[8]1053</section>
1054
1055</section>
1056
[29]1057<section title="IANA Considerations" anchor="IANA.considerations">
[253]1058<section title="Message Header Registration" anchor="message.header.registration">
[290]1059<t>
1060   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1061   with the permanent registrations below (see <xref target="RFC3864"/>):
1062</t>
[253]1063<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[290]1064<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
[253]1065   <ttcol>Header Field Name</ttcol>
1066   <ttcol>Protocol</ttcol>
1067   <ttcol>Status</ttcol>
1068   <ttcol>Reference</ttcol>
1069
1070   <c>ETag</c>
1071   <c>http</c>
1072   <c>standard</c>
1073   <c>
1074      <xref target="header.etag"/>
1075   </c>
1076   <c>If-Match</c>
1077   <c>http</c>
1078   <c>standard</c>
1079   <c>
1080      <xref target="header.if-match"/>
1081   </c>
1082   <c>If-Modified-Since</c>
1083   <c>http</c>
1084   <c>standard</c>
1085   <c>
1086      <xref target="header.if-modified-since"/>
1087   </c>
1088   <c>If-None-Match</c>
1089   <c>http</c>
1090   <c>standard</c>
1091   <c>
1092      <xref target="header.if-none-match"/>
1093   </c>
1094   <c>If-Unmodified-Since</c>
1095   <c>http</c>
1096   <c>standard</c>
1097   <c>
1098      <xref target="header.if-unmodified-since"/>
1099   </c>
1100   <c>Last-Modified</c>
1101   <c>http</c>
1102   <c>standard</c>
1103   <c>
1104      <xref target="header.last-modified"/>
1105   </c>
1106</texttable>
[290]1107<!--(END)-->
[253]1108<t>
[290]1109   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1110</t>
[8]1111</section>
[253]1112</section>
[8]1113
1114<section title="Security Considerations" anchor="security.considerations">
1115<t>
[29]1116   No additional security considerations have been identified beyond
1117   those applicable to HTTP in general &messaging;.
[8]1118</t>
1119</section>
1120
1121<section title="Acknowledgments" anchor="ack">
1122</section>
1123</middle>
1124<back>
[119]1125
1126<references title="Normative References">
1127
[31]1128<reference anchor="Part1">
[119]1129  <front>
1130    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1131    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1132      <organization abbrev="Day Software">Day Software</organization>
1133      <address><email>fielding@gbiv.com</email></address>
1134    </author>
1135    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1136      <organization>One Laptop per Child</organization>
1137      <address><email>jg@laptop.org</email></address>
1138    </author>
1139    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1140      <organization abbrev="HP">Hewlett-Packard Company</organization>
1141      <address><email>JeffMogul@acm.org</email></address>
1142    </author>
1143    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1144      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1145      <address><email>henrikn@microsoft.com</email></address>
1146    </author>
1147    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1148      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1149      <address><email>LMM@acm.org</email></address>
1150    </author>
1151    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1152      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1153      <address><email>paulle@microsoft.com</email></address>
1154    </author>
1155    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1156      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1157      <address><email>timbl@w3.org</email></address>
1158    </author>
1159    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1160      <organization abbrev="W3C">World Wide Web Consortium</organization>
1161      <address><email>ylafon@w3.org</email></address>
1162    </author>
1163    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1164      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1165      <address><email>julian.reschke@greenbytes.de</email></address>
1166    </author>
1167    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1168  </front>
1169  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1170  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
[31]1171</reference>
1172
1173<reference anchor="Part5">
[119]1174  <front>
1175    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1176    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1177      <organization abbrev="Day Software">Day Software</organization>
1178      <address><email>fielding@gbiv.com</email></address>
1179    </author>
1180    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1181      <organization>One Laptop per Child</organization>
1182      <address><email>jg@laptop.org</email></address>
1183    </author>
1184    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1185      <organization abbrev="HP">Hewlett-Packard Company</organization>
1186      <address><email>JeffMogul@acm.org</email></address>
1187    </author>
1188    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1189      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1190      <address><email>henrikn@microsoft.com</email></address>
1191    </author>
1192    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1193      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1194      <address><email>LMM@acm.org</email></address>
1195    </author>
1196    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1197      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1198      <address><email>paulle@microsoft.com</email></address>
1199    </author>
1200    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1201      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1202      <address><email>timbl@w3.org</email></address>
1203    </author>
1204    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1205      <organization abbrev="W3C">World Wide Web Consortium</organization>
1206      <address><email>ylafon@w3.org</email></address>
1207    </author>
1208    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1209      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1210      <address><email>julian.reschke@greenbytes.de</email></address>
1211    </author>
1212    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1213  </front>
1214  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1215  <x:source href="p5-range.xml" basename="p5-range"/>
[31]1216</reference>
1217
1218<reference anchor="Part6">
[119]1219  <front>
1220    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1221    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1222      <organization abbrev="Day Software">Day Software</organization>
1223      <address><email>fielding@gbiv.com</email></address>
1224    </author>
1225    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1226      <organization>One Laptop per Child</organization>
1227      <address><email>jg@laptop.org</email></address>
1228    </author>
1229    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1230      <organization abbrev="HP">Hewlett-Packard Company</organization>
1231      <address><email>JeffMogul@acm.org</email></address>
1232    </author>
1233    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1234      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1235      <address><email>henrikn@microsoft.com</email></address>
1236    </author>
1237    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1238      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1239      <address><email>LMM@acm.org</email></address>
1240    </author>
1241    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1242      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1243      <address><email>paulle@microsoft.com</email></address>
1244    </author>
1245    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1246      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1247      <address><email>timbl@w3.org</email></address>
1248    </author>
1249    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1250      <organization abbrev="W3C">World Wide Web Consortium</organization>
1251      <address><email>ylafon@w3.org</email></address>
1252    </author>
1253    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1254      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1255      <address><email>julian.reschke@greenbytes.de</email></address>
1256    </author>
1257    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1258  </front>
1259  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1260  <x:source href="p6-cache.xml" basename="p6-cache"/>
[31]1261</reference>
1262
[96]1263<reference anchor="RFC2119">
1264  <front>
1265    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1266    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1267      <organization>Harvard University</organization>
1268      <address><email>sob@harvard.edu</email></address>
1269    </author>
1270    <date month="March" year="1997"/>
1271  </front>
1272  <seriesInfo name="BCP" value="14"/>
1273  <seriesInfo name="RFC" value="2119"/>
1274</reference>
1275
[425]1276<reference anchor="RFC5234">
1277  <front>
1278    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1279    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1280      <organization>Brandenburg InternetWorking</organization>
1281      <address>
1282      <postal>
1283      <street>675 Spruce Dr.</street>
1284      <city>Sunnyvale</city>
1285      <region>CA</region>
1286      <code>94086</code>
1287      <country>US</country></postal>
1288      <phone>+1.408.246.8253</phone>
1289      <email>dcrocker@bbiw.net</email></address> 
1290    </author>
1291    <author initials="P." surname="Overell" fullname="Paul Overell">
1292      <organization>THUS plc.</organization>
1293      <address>
1294      <postal>
1295      <street>1/2 Berkeley Square</street>
1296      <street>99 Berkely Street</street>
1297      <city>Glasgow</city>
1298      <code>G3 7HR</code>
1299      <country>UK</country></postal>
1300      <email>paul.overell@thus.net</email></address>
1301    </author>
1302    <date month="January" year="2008"/>
1303  </front>
1304  <seriesInfo name="STD" value="68"/>
1305  <seriesInfo name="RFC" value="5234"/>
1306</reference>
1307
[119]1308</references>
1309
1310<references title="Informative References">
1311
1312<reference anchor="RFC2616">
1313  <front>
1314    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1315    <author initials="R." surname="Fielding" fullname="R. Fielding">
1316      <organization>University of California, Irvine</organization>
1317      <address><email>fielding@ics.uci.edu</email></address>
1318    </author>
1319    <author initials="J." surname="Gettys" fullname="J. Gettys">
1320      <organization>W3C</organization>
1321      <address><email>jg@w3.org</email></address>
1322    </author>
1323    <author initials="J." surname="Mogul" fullname="J. Mogul">
1324      <organization>Compaq Computer Corporation</organization>
1325      <address><email>mogul@wrl.dec.com</email></address>
1326    </author>
1327    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1328      <organization>MIT Laboratory for Computer Science</organization>
1329      <address><email>frystyk@w3.org</email></address>
1330    </author>
1331    <author initials="L." surname="Masinter" fullname="L. Masinter">
1332      <organization>Xerox Corporation</organization>
1333      <address><email>masinter@parc.xerox.com</email></address>
1334    </author>
1335    <author initials="P." surname="Leach" fullname="P. Leach">
1336      <organization>Microsoft Corporation</organization>
1337      <address><email>paulle@microsoft.com</email></address>
1338    </author>
1339    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1340      <organization>W3C</organization>
1341      <address><email>timbl@w3.org</email></address>
1342    </author>
1343    <date month="June" year="1999"/>
1344  </front>
1345  <seriesInfo name="RFC" value="2616"/>
1346</reference>
1347
[253]1348<reference anchor='RFC3864'>
1349  <front>
1350    <title>Registration Procedures for Message Header Fields</title>
1351    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1352      <organization>Nine by Nine</organization>
1353      <address><email>GK-IETF@ninebynine.org</email></address>
1354    </author>
1355    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1356      <organization>BEA Systems</organization>
1357      <address><email>mnot@pobox.com</email></address>
1358    </author>
1359    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1360      <organization>HP Labs</organization>
1361      <address><email>JeffMogul@acm.org</email></address>
1362    </author>
1363    <date year='2004' month='September' />
1364  </front>
1365  <seriesInfo name='BCP' value='90' />
1366  <seriesInfo name='RFC' value='3864' />
1367</reference>
1368
[8]1369</references>
[99]1370
1371<section title="Compatibility with Previous Versions" anchor="compatibility">
1372
1373<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
[245]1374<t>
1375  Allow weak entity tags in all requests except range requests (Sections
1376  <xref target="weak.and.strong.validators" format="counter"/> and
1377  <xref target="header.if-none-match" format="counter"/>).
1378</t>
[99]1379</section>
1380
1381</section>
1382
[427]1383<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1384<figure>
1385<artwork type="abnf" name="p4-conditional.parsed-abnf">
1386<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1387<x:ref>ETag-v</x:ref> = entity-tag
1388
1389<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1390
1391<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1392<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1393 entity-tag ] ) )
1394<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1395<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1396<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1397<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1398 entity-tag ] ) )
1399<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1400 If-Unmodified-Since-v
1401<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1402
1403<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1404<x:ref>Last-Modified-v</x:ref> = HTTP-date
1405
1406<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1407
1408<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1409
1410<x:ref>opaque-tag</x:ref> = quoted-string
1411
1412<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1413
1414<x:ref>weak</x:ref> = "W/"
1415
1416; ETag defined but not used
1417; If-Match defined but not used
1418; If-Modified-Since defined but not used
1419; If-None-Match defined but not used
1420; If-Unmodified-Since defined but not used
1421; Last-Modified defined but not used
1422
1423
[421]1424</artwork>
[427]1425</figure>
[421]1426</section>
1427
[252]1428<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
[115]1429
1430<section title="Since RFC2616">
1431<t>
1432  Extracted relevant partitions from <xref target="RFC2616"/>.
1433</t>
1434</section>
1435
1436<section title="Since draft-ietf-httpbis-p4-conditional-00">
1437<t>
[152]1438  Closed issues:
1439  <list style="symbols"> 
1440    <t>
[324]1441      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
[152]1442      "Normative and Informative references"
1443    </t>
1444  </list>
1445</t>
1446<t>
[116]1447  Other changes:
1448  <list style="symbols"> 
1449    <t>
1450      Move definitions of 304 and 412 condition codes from Part2.
1451    </t>
1452  </list>
[115]1453</t>
1454</section>
1455
[170]1456<section title="Since draft-ietf-httpbis-p4-conditional-01">
1457<t>
[324]1458  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
[205]1459  <list style="symbols"> 
1460    <t>
1461      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1462    </t>
1463  </list>
[170]1464</t>
[115]1465</section>
1466
[252]1467<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
[228]1468<t>
[245]1469  Closed issues:
1470  <list style="symbols"> 
1471    <t>
[324]1472      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
[245]1473      "Weak ETags on non-GET requests"
1474    </t>
1475  </list>
[228]1476</t>
[253]1477<t>
[324]1478  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
[253]1479  <list style="symbols"> 
1480    <t>
1481      Reference RFC 3984, and update header registrations for headers defined
1482      in this document.
1483    </t>
1484  </list>
1485</t>
[170]1486</section>
1487
[267]1488<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1489<t>
[298]1490  Closed issues:
1491  <list style="symbols"> 
1492    <t>
[324]1493      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
[298]1494      "Examples for ETag matching"
1495    </t>
[302]1496    <t>
[324]1497      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
[302]1498      "'entity value' undefined"
1499    </t>
[306]1500    <t>
[324]1501      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
[306]1502      "bogus 2068 Date header reference"
1503    </t>
[298]1504  </list>
[267]1505</t>
[228]1506</section>
1507
[323]1508<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1509<t>
[334]1510  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1511  <list style="symbols"> 
1512    <t>
1513      Use "/" instead of "|" for alternatives.
1514    </t>
[362]1515    <t>
1516      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1517      whitespace ("OWS") and required whitespace ("RWS").
1518    </t>
1519    <t>
1520      Rewrite ABNFs to spell out whitespace rules, factor out
1521      header value format definitions.
1522    </t>
[334]1523  </list>
[323]1524</t>
[267]1525</section>
1526
[382]1527<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1528<t>
[421]1529  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1530  <list style="symbols"> 
1531    <t>
[424]1532      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
[421]1533    </t>
1534  </list>
[382]1535</t>
[323]1536</section>
1537
[382]1538</section>
1539
[8]1540</back>
1541</rfc>
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